Stability of the formation of the chevron structure

A simple model of the formation of the chevron structure and tilted layer structure in the smectic C liquid crystal phase from the bookshelf structure in the smectic A phase is considered. Energetic considerations of this system indicate that in the absence of layer pinning forces at the surface, a transition to the tilted structure is expected. However, combining the model with 'weak' surface positional anchoring effects allows the chevron structure to form.     

Silver-cemented frit formation for the stabilization of the packing structure in the microchannel of electrochromatographic microchips

A simple but effective frit formation technique was developed to stabilize the packing structure inside the microchannel of capillary electrochromatographic microchips, utilizing the electroless plating technique. A Ag(NH(3))(2)(+) solution was allowed to diffuse through the colloidal silica packing in the microchannel from the reservoir of the microchip for a limited amount of time, and then it was reduced by an excess amount of formaldehyde solution. A frit structure of ~70 μm in length was formed at the entrance of the microchannel without clogging when treated with 1mM Ag(NH(3))(2)(+) ion and formaldehyde for 30s and 150 s, respectively. The formation of the frit structure was confirmed by a scanning electron microscopy. The stability of the packing structure was tested rigorously and then confirmed by applying alternating electroosmotic flows back and forth with pulsed potential steps on both sides of the frit structure. The effect of the treatment on the electrochromatograms was evaluated after the microchips were repeatedly used and stored for a long period of time. The results indicated that the silver-cemented frit structure extended the lifetime of the fully packed CEC microchips distinctly.     

Characterization of the benzene-insoluble fraction of the hydrolyzate of phenyltrimethoxysilanes in the presence of benzyltrimethylammonium hydroxide

The insoluble fraction obtained from the hydrolysis and condensation of (4-dimethylamino-, 2-methyl-, and nonsubstituted)phenyltrimethoxysilanes in the presence of benzyltrimethylammonium hydroxide in benzene was characterized. IR, ¹H NMR, and ²⁹Si NMR suggested that the insoluble fraction was composed of RSi(O )₃, that is, a T³ structure. X-ray diffraction indicated the presence of a long-range ordered structure composed of a mixture of crystals. Raman spectroscopy strongly suggested a cage structure by the presence of a ring-opening vibration assignable to a cubic structure at 475–482 cm⁻¹. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4587–4597, 2004     

Effect of γ-irradiation on the molecular-topological structure of the polyethylene matrix of glass-reinforced plastic

The molecular-topological structure of polyethylene (PE) and a PE matrix in a glass-reinforced plastic (GRP) after γ-irradiation of the plastic was studied by means of thermomechanical spectroscopy. The four-block topological structure of unirradiated PE (one amorphous block and three crystalline phases with different initial melting temperatures) is transformed into a three-block structure in the GRP already at a minimal radiation dose of 25 kGy. The intermediate crystalline phase disappears under these conditions, the molecular relaxation characteristics in all topological blocks alter, and chemical branching points appear in the pseudonetwork structure of the amorphous matrix block.     

On the mechanism of the hofmeister effect

Vibrational sum frequency spectroscopy was used to probe fatty amine monolayers spread on various electrolyte solutions. The spectra revealed ion specific changes in both monolayer ordering and water structure with the former following the Hofmeister series. Separate measurements of the surface potential as a function of ion tracked closely to changes in alkyl chain structure, but less closely to changes in water structure. The disruption of the monolayer ordering could be ascribed to the relative ability of the ions to penetrate past the hydrophilic surface of the monolayer's headgroups and into the more hydrophobic portion of the thin film. The corresponding trends observed in the surface water structure showed significant deviations from the Hofmeister series, leading to the conclusion that the changes in surface water structure, often credited with being the origin of Hofmeister effects, are probably not of primary importance. On the other hand, dispersion forces almost certainly play a large role in the order of the Hofmeister series.     

Preparation of echinus-like SiO2@Ag structures with the aid of the HCP phase

A novel echinus-like hybrid structure composed of a SiO(2)@Ag core-shell and many silver nanorods was synthesized. These silver rods have a rare HCP structure, while the initial silver nuclei have a FCC structure. The rapid and free growth of silver nuclei resulted in the appearance of the HCP phase.     

Structure of the xylan from the stems of the cotton plant

Summary 1. The structure of the aldobiuronic acid isolated from the xylan of cotton-plant stems has been established by its reduction and the hydrolysis of the reduced product.2. From a study of the products of the hydrolysis of the methylated xylan from cotton-plant stems the positions of attachment of the side chains in the molecule of the xylan and their structure have been determined, and a most probable structure for the xylan molecule has been put forward.V. I. Lenin Tashkent State University. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 432–435, July–August, 1974.     

Experimental and theoretical investigation on the relative stability of the PdS2- and pyrite-type structures of PdSe2

Under ambient condition PdSe2 has the PdS2-type structure. The crystal structure of PdSe2 under pressure (up to 30 GPa) was investigated at room temperature by X-ray diffraction in an energy-dispersive configuration using a diamond anvil cell with a mixture of water/ethanol/methanol as a pressure transmitting medium. A reversible structural transition from the PdS2-type to the pyrite-type structure occurs around 10 GPa, and the applied pressure reduces the spacing between adjacent 2/proportional to [PdSe2] layers of the PdS2-type structure to form the three-dimensional lattice of the pyrite-type structure. First principles and extended Hückel electronic band structure calculations were carried out to confirm the observed pressure-induced structural changes. We also examined why the isoelectronic analogues NiSe2 and PtSe2 adopt structures different from the PdS2-type structure on the basis of qualitative electronic structure considerations.     

An alternative description of the paulingite structure

The paulingite structure can be described as an invariant transformation of the gismondine structure. Packets of a fourling construction of the gismondine structures, which is closely related to the merlinoite structure, are used to build the structure of paulingite as an interpenetrating sixling.     

Dependence of the conformational parameters of polyimides on the chemical structure of the chain

The flexibility of some polyimide chains has been studied theoretically as a function of the chemical structure of the repeat unit. It has been shown that the thermodynamic flexibility can be varied over a wide range by the choice of the unit structure. For three polyimides, as an example, the flexibility of cyclochain structures approximates that for free rotation.     

State of the art in the electrochemical characterization of the surface structure of shape-controlled Pt,Au, and Pd nanoparticles

The application of shape-controlled metal nanoparticles in electrocatalysis has improved significantly the activity, selectivity, and even the stability of many relevant electrocatalytic reactions. It is well accepted that, by controlling the shape of the nanoparticles, it is possible to provide nanoparticles with a preferential surface structure. However, to fully understand the capabilities of these nanomaterials, it is extremely relevant to correlate shape, surface structure, and electrocatalytic reactivity. Particularly, establishing the correlations between surface structure and reactivity is the key point to be studied and understood. Consequently, having tools to characterize the surface structure of these nanoparticles is of critical importance. In this short review, we discuss about the progress in the in situ characterization of the surface structure of shaped Pt, Au, and Pd nanoparticles by electrochemical probes. The results here included clearly demonstrate the potentialities of the electrochemical tools to gain detailed information of the surface structure of these shaped nanomaterials.     

Confirmation of the Structures of Lutein and Zeaxanthin

A series of new esters of lutein ( 1a ) have been prepared with the aim of confirming the structure of lutein via an X‐ray crystal‐structure analysis. Although well crystallized, only one of the derivatives, the (?)‐(1R)‐menthyl carbonate ( 1i ) proved to be useful for a complete structure analysis. The same derivative of zeaxanthin ( 2a ) also allowed its crystal structure to be determined. Both analyses represent the first successful X‐ray crystal structure analyses of the most important xanthophylls. At the same time, they confirm both the constitution and absolute configuration of 1a and 2a that had been deduced earlier by classical methods.     

Total synthesis of the putative structure of stemonidine: the definitive proof of misassignment

[structure: see text] The total synthesis of the putative structure of the Stemona alkaloid stemonidine has been completed. The key transformations include a 1,3-dipolar cycloaddition of a chiral nitrone derived from (S)-prolinol and a spirolactonization process involving the generation of the critical stereocenter. The NMR data of the synthetic material do not match those reported for the natural product. It is concluded that the structure assigned to stemonidine is incorrect, and it must be reassigned as stemospironine.     

Influence of solvent structure on the conformation of the native DNA molecule

The investigation of the factors determining the formation and stability of the higher biopolymers structures is one of the most important trends of the development of molecular biophysics. A feature common to most macromolecular systems under physiological conditions is that they function in an aqueous environment. Thus, it is natural to assume that the peculiarities of biological macromolecules structures and their functional activity as well are closely related to the specific properties of such a unique solvent as water. The investigations of the conformational changes of biopolymer, induced by dehydration of the macromolecule, give information about the nature of the forces stabilizing its structure. The dehydration of the macromolecule in solution can be attained by addition of a nonaqueous cosolvent. Generally low-molecular-weight aliphatic alcohols, amides, and amines are used as a nonaqueous component. At present a vast number of experimental and theoretical data concerning the properties of water and aqueous systems are available. The specificity of water as a solvent arises primarily from the spatial hydrogen-bonded structure. The addition of a nonaqueous component exerts changes in this structure, which evolve to the singularities of the physical characteristics of water-nonelectrolyte mixtures. It is generally assumed that nonelectrolytes may be divided, according to their effect on the spatial water structure, largely into two basic classes: (1) the structure makers, i.e., the compounds of aliphatic alcohols type; (2) the structure breakers, i.e., the compounds of urea type. The agents belonging to the first class show a stabilizing effect in the range of low nonelectrolyte content. At a certain critical concentration, C_crit, characteristic of each substance, the nonaqueous solute molecules leave the cavities of the spatial water structure which leads to a disruption of the latter. The agents belonging to the second class exert a structure-breaking effect even in the range of extremely low concentrations, which arises from their high competitive ability for hydrogen bonding.     

X-ray spectroscopic and diffraction study of the structure of the active species in the Ni(II)-catalyzed polymerization of isocyanides.

The structure of the active complex in the Ni-catalyzed polymerization of isocyanides to give polyisocyanides is investigated. It is shown by X-ray absorption spectroscopy (XAS), including EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure), and single-crystal X-ray diffraction, to contain a carbene-like ligand. This is the first structural characterization of a crucial intermediate in the so-called merry-go-round mechanism for Ni-catalyzed isocyanide polymerization.     

A study of the globulins of cotton seeds XX. Stability of the quaternary structure of the 11S-globulin

The 11S-globulin has a complex stable quaternary structure in the formation of which three types of subunits participate. The aim of the work was to determine what forces form and stabilize the quaternary structure of this protein. The products of the maleylation and acetylation of the 11S-globulin and also the stability of the native 11S-globulin in the presence of ionic and nonionic detergents were investigated by disk electrophoresis. It was shown that hydrophobic interactions play a deciding role in the formation and stabilization of the quaternary structure of the 11S-globulin.     

Unveiling the complex pattern of intermolecular interactions responsible for the stability of the DNA duplex

Herein, we provide new insights into the intermolecular interactions responsible for the intrinsic stability of the duplex structure of a large portion of human B-DNA by using advanced quantum mechanical methods. Our results indicate that (i) the effect of non-neighboring bases on the inter-strand interaction is negligibly small, (ii) London dispersion effects are essential for the stability of the duplex structure, (iii) the largest contribution to the stability of the duplex structure is the Watson–Crick base pairing – consistent with previous computational investigations, (iv) the effect of stacking between adjacent bases is relatively small but still essential for the duplex structure stability and (v) there are no cooperativity effects between intra-strand stacking and inter-strand base pairing interactions. These results are consistent with atomic force microscope measurements and provide the first theoretical validation of nearest neighbor approaches for predicting thermodynamic data of arbitrary DNA sequences.

Advanced electronic structure methods provide a new insight into the intermolecular interactions responsible for the intrinsic stability of the duplex structure of human DNA.     

Determination of the Absolute Configuration of the Pseudo‐Symmetric Natural Product Elatenyne by the Crystalline Sponge Method

Elatenyne is a marine natural product that was isolated in 1986. Despite its simple 2,2′‐bifuranyl backbone, its relative structure was only recently determined. The absolute configuration of elatenyne has still not been unequivocally confirmed because of its pseudo‐meso core structure, which results in a specific rotation, [α]_D , of almost zero. In this work, the structure of natural elatenyne was determined by the crystalline sponge method and the use of a porous coordination network (a crystalline sponge) capable of absorbing organic guests; in the sponge, the absorbed guests are ordered and crystallographically observable. The crystalline sponge could differentiate between the two very similar alkyl side chains, and the absolute structure of elatenyne was thus reliably determined. The total amount required for the experiments was only approximately 100 μg, and the majority (95 μg) could be recovered after the experiments.